The present invention relates to an underfill sealing agent to be used for mounting, onto a wiring board, a semiconductor device such as a chip size package (Chip Size/Scale Package; hereinafter abbreviated as a CSP) and a ball grid array (Ball Grid Array; hereinafter abbreviated as a BGA), which has a semiconductor element such as an LSI and a bear IC chip disposed on a carrier base.
Recently, with the propagation of small size electronic appliances such as a portable phone, a camera-integrated VTR, and a laptop personal computer, downsizing of an LSI device and an IC chip has been demanded. Further, CSPs and BGAs are propagated for achieving a small size like a bear chip and improving the characteristics while maintaining the characteristics of the package, which include protecting the semiconductor bear chip such as an LSI and facilitating a test.
The CSP and the BGA are connected with a wiring on a wiring board by a solder or the like. However, in some cases the connection reliability between the board and the CSP or BGA cannot be maintained in the case a temperature cycle is applied after the mounting operation. In general, after mounting the CSP and the BGA on the wiring board, a sealing resin is introduced into the gap between the CSP or BGA and the board (underfill sealing) for alleviating the stress by the temperature cycle and improving the heat shock resistance property, so as to improve the reliability of the electric connection. Moreover, the underfill sealing agent is used also as a reinforcing agent for fall prevention of the CSP or BGA due to impact by dropping or the like.
As the underfill sealing agents, thermosetting type epoxy resins, acrylic resins (Japanese Patent No. 2,746,035, JP-A-10-101906 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-10-158366, JP-A-10-64932) and the like have been conventionally used. However, a problem is involved in that since a thermosetting resin is used as the sealing material, in the case where a defect of an LSI on a CSP or BGA, a defect of the connection between a CSP or BGA and a wiring board, or the like is found after mounting the CSP or BGA on the wiring board, it is extremely difficult to peeling off the thermosetting resin for replacement of the CSP or BGA.
Moreover, JP-A-5-102343 discloses a mounting method comprising fixing and connecting a bear chip on a wiring board using a photo-curing adhesive, and eliminating it in the case of having a defect. A problem is involved in that since the photo-curing adhesive is used, the method is limited only to a transparent substrate that allows light irradiation, such as a glass.
Furthermore, JP-A-6-69280 discloses a method comprising fixing and connecting a bear chip and a substrate using a resin to be cured at a predetermined temperature, and detaching the bear chip, when it has a defect, by softening the resin at a temperature higher than the predetermined temperature. However, any specific disclosure on the adhesive is not given in this publication. A method satisfying both the reliability and the repair characteristic has not been known.
Then, for peeling off a substrate from the above-mentioned curing resin, a peeling operation has been made by soaking in an organic solvent or the like. However, the performance inherent to the adhesive is deteriorated if the peeling property (repair property) is improved, and the peeling property is deteriorated if the bonding property and the durability of the adhesive are improved. An adhesive capable of combining the inherent function as an adhesive and the peeling property has not yet been known. In view of this, in JP-A-6-77264, a method of removing a resin residue by irradiating an electromagnetic wave, instead of peeling by swelling or dissolution using a solvent, is adopted. According to the method, not only the equipment becomes large-scale but also it merely removes the adhesive residue at most, and thus the adhesive peeling property itself cannot be improved dramatically.
In addition, JP-A-5-251516 discloses a mounting method comprising connecting and fixing a bear chip on a wiring board using a bisphenol A type epoxy resin, and removing it in the case of a defect. However, according to the method, detachment of the chip is not necessarily easy. In the case a method of cutting the chip by a milling process is adopted, since the chip itself is cut out mechanically, a problem arises in that the chip cannot be reused even in the case the chip is normal.
As an adhesive provided with a peeling property (repair property) for solving the above-mentioned problems, JP-A-10-204259 discloses a thermosetting resin composition for underfill sealing capable of heat-curing in a short time by adding a plasticizer to a one liquid type or two liquid type epoxy resin, capable of connecting a semiconductor device such as a CSP and a BGA with a wiring board, having the excellent heat shock resistance (temperature cycling property), and capable of easily detaching the CSP or BGA when a defect is found. However, according to the method, since a plasticizer is used, a problem is involved in that the resin strength, i.e., the durability, the heat resistance, and the heat cycling resistance is deteriorated, and the environment is contaminated due to bleeding of the plasticizer from its cured product.
Accordingly, in order to solve the above-mentioned problems, an object of the present invention is to provide an epoxy resin composition for underfill sealing, which is capable of heat-curing in a short time to provide good productivity, is capable of surely connecting a semiconductor device such as a CSP and a BGA on a wiring board without giving an adverse effect on each part disposed on the wiring board owing to heat-curing at a relatively low temperature, has excellent heat shock resistance (temperature cycling property) and impact resistance after curing, is free from bleeding of contaminants from its cured product, and is capable of easily detaching the CSP or BGA from the wiring board when a defect is found, so as to enable reuse of a normal wiring board or a semiconductor device.
The present invention provides an epoxy resin composition comprising, as main components, (a) 100 parts by weight of a polyfunctional epoxy resin which stays liquid at an ordinary temperature and has two or more glycidyl groups in the molecule thereof, (b) 3 to 80 parts by weight of a curing agent, and (c) 1 to 100 parts by weight of a modified epoxy resin.
Moreover, the present invention also relates to an underfill sealing method using the above-mentioned composition, and a circuit board produced utilizing the method.
Despite the fact that the epoxy resin composition of the present invention cures in a short time or at a relatively low temperature, it has excellent heat shock resistance (temperature cycling property) and impact resistance of its cured product. Besides, the cured product has a nature to be easily torn by heating and applying a force. Furthermore, the composition enables that its cured product adhered on a wiring board or the like can also be removed easily by heating.
The use of the thermosetting resin composition makes it possible to carry out heat-curing in a short time to give a good productivity, and makes it possible to connect a semiconductor device such as a CSP and a BGA surely on a wiring board without causing an adverse effect on each part on the wiring board by a relatively low temperature heat-curing. The semiconductor mounting structure after the connection has excellent heat shock resistance (temperature cycling property) and impact resistance. Moreover, bleeding of contaminants is not generated from its cured product. Therefore, since the semiconductor device can be detached easily in the case where a defect is found in the electric connection or the like, the semiconductor device, the wiring board or the like can be reused so that improvement of the yield in the production step, and reduction of the production cost can be achieved.